1. Field of the Invention
The invention relates to the domain of materials demonstrating self-cleaning properties and/or ease of cleaning. It specifically concerns architectural materials comprising a substrate at least partially organic, such as PVC, polyethylene, polypropylene, painted wood or painted aluminum, coated with at least with one thin layer able to protect the said substrate against oxidation reactions via radicals, as well as the process by which such materials may be obtained.
2. Brief Description of the Prior Art
Some materials, specifically those used in architecture, are generally subjected to dust and atmospheric pollution, and suffer deterioration of their surface appearance with the passage of time. In order to avoid excessively frequent and costly cleaning, some materials which possess self-cleaning properties and/or which are easy to clean, have been developed. Some of these make use of the photocatalytic and photo-induced superhydrophilic properties of titanium dioxide (TiO2), being coated with one or several layers comprising this oxide. The latter, in particular when crystallized in the anatase form, has the ability, when irradiated with ultraviolet light (from the sun, for example), to become superhydrophilic and to catalyze radical reactions leading to degradation of organic dirt: these two properties are independent. Specifically, the former property prevents the formation of droplets of water which leave stains due to mineral deposits when they dry, and in fact encourages wetting of the surface by rain water and the consequent removal of dirt. The material thus obtained therefore presents the ability to self-clean under the action of sunlight and rain water. Patent application EP-A-0 850 204 describes this type of application for glass windows.
In the case of architectural materials whose surfaces comprise organic compounds such as polyvinyl chloride (PVC), polyethylene, polypropylene, painted wood or painted aluminum, it is not possible to deposit a photocatalytic layer directly onto the material since the photocatalytic coating would degrade these organic compounds under the influence of radical oxidation reactions. It is therefore proposed that mineral barrier layers should be used to protect these materials. Patent application EP-A-1 498 176 describes for example 30 micrometer thick barrier layers based on SiO2, ZrO2 etc., which protect the extrusions of PVC windows from the photocatalytic effects of a TiO2 coating. These barrier layers are obtained at low temperature by a sol-gel type of process, in which the precursors in solution in a solvent (the sol) are deposited onto the substrate; when they gel they form a solid coating on the surface of the said substrate.